1. Field of the Invention
The present invention relates to valve assemblies for operating hydraulically powered machinery; and more particularly to such valve assemblies that produce a pressure signal which controls a variable displacement hydraulic pump.
2. Description of the Related Art
The speed of a hydraulically driven working member on a machine depends upon the cross-sectional area of principal narrowed orifices of the hydraulic system and the pressure drop across those orifices. To facilitate control, pressure compensating hydraulic control systems have been designed to eliminate the pressure drop. These previous control systems include load sense conduits which transmit the pressure at the valve workports to the input of a variable displacement hydraulic pump supplying pressurized hydraulic fluid in the system. The resulting self adjustment of the pump output provides an approximately constant pressure drop across a control orifice whose cross-sectional area can be controlled by the machine operator. This facilitates control because, with the pressure drop held constant, the speed of movement of the working member is determined only by the cross-sectional area of the orifice.
One such system is disclosed in U.S. Pat. No. 5,715,865 entitled “Pressure Compensating Hydraulic Control Valve System” in which a separate valve section controls the flow of hydraulic fluid from the pump to each hydraulic actuator that drive a working member. The valve sections are of a type in which the greatest load pressure acting on the hydraulic actuators is sensed to provide a load sense pressure which is transmitted to the control input port of the pump. The greatest load pressure is determined by daisy chain of shuttle valves that receives the load pressure from all the valve sections.
Each valve section includes a control valve, with a variable metering orifice, and a separate pressure compensating valve. The output pressure from the pump is applied to one side of the metering orifice and the pressure compensating valve at the other side of the metering orifice, responds to the load sense pressure, so that the pressure drop across the metering orifice is held substantially constant.
While this system is effective, it requires a separate pressure compensating valve and a shuttle valve in each valve section, in addition to the control valve that has the metering orifice. These additional components add cost and complexity to the hydraulic system, which can be a important consideration for less expensive machines. Thus, there is need for a less expensive and less complex technique for performing this function.